1. Field of the Invention
The present invention relates to an image reading apparatus for reading images formed on sheet originals, such as a scanner, a copying machine, a printer, and a facsimile machine. The present invention further relates to an image forming apparatus having the image reading apparatus.
2. Description of the Related Art
Conventionally, image reading apparatuses each provided in an apparatus, such as a digital copying machine, a printer, or a facsimile machine, includes those of the type that has an auto document feeder (ADF), which is an example of sheet automatic feeders, mounted to be openable/closable therein to automatically feed sheet originals to the image reading apparatus.
Reading techniques of the image reading apparatus having the ADF include those of the type that reads an image on an original automatically fed by the ADF in the following manner. A reading unit provided below the ADF is moved by a motor to a preset position and stopped. Thereafter, the original fed at a predetermined speed is scanned using light emitted by a lamp unit, and light reflected off therefrom is detected by the reading unit to read the image (the read hereinbelow will be referred to as a “motional read.”
FIGS. 8 and 9 each show configurations near a read position of a conventional ADF that employs the motional read.
The configuration shown in FIG. 8 has transport roller pairs (402 and 403) disposed, respectively, on an upstream side and a downstream side of a read position α. In addition, the configuration has a platen glass 404 provided as an original base plate substantially on an upper portion of the read position α, and a guide member 405 that forms a predetermined gap with the platen glass 404. While an original P is restrained by the gap, a reading unit 401 is moved to the read position α, and the original P is transported at a predetermined speed over he reading unit 401 positioned at the read position α. In this manner, speed variations during original transport are restrained, and the image read is performed.
FIG. 9 shows another configuration that, in lieu of the guide member 405, has a platen roller 406 rotatable by driving means (not shown). In this configuration, an original is transported while being pressed against the platen glass 404 with a gap narrower than the gap between the guide member 405 and the platen glass 404.                Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 6-291938;        Patent Document 2: JP-A No. 2001-066713; and        Patent Document 3: JP-A No. 2002-232643.        
However, in the prior art described above, problems as described below take place.
In the configuration shown in FIG. 8, the guide member 405 has a sliding resistance during the original transport, so that the gap between the guide member 405 and the platen glass 404 should be at least about 1 mm. This gap causes the original P to be bent and transported in the bent state. As such, light quantity reduces in a portion where the sheet floats from the platen glass 404 in the bent portion. This reduces the readable region having a luminance level suitable for reading. For this reason, in the case the read position a is off the readable region, a case can occur in which image quality is degraded.
Another configuration is shown in FIG. 9. In this configuration, an original P is transported in close contact with a platen roller 406 serving as a transport rotary member, and the original P can be intensively biased onto the platen glass 404 for transport. On the other hand, however, problems arise in the case that the platen roller 406 is designed to have a small curvature in order to implement apparatus miniaturization. Specifically, when a read position β is shifted from the center of the platen roller 406, the original P leaves the platen glass 404. This reduces light quantity, thereby disabling a luminance level suitable for reading to be maintained. As such, the readable region is limited to the vicinity of the center of the platen roller 406. Consequently, as in the case of the guide member 405, when the read position β is shifted from the readable region, a case can occur in which image quality is degraded.